DomainKeys For Email Deliverability
This is an authentication standard that is designed to verify the DNS domain of email sender and the message integrity. Technically, it validates the domain name identity associated with a message through cryptographic authentication. All outgoing emails are digitally signed with a private encryption key to match a public key that is published in the sender’s DNS record. It is used by Gmail, Yahoo, SBCGlobal, British Telecom, Rogers Cable, Rocket Mail, etc. It is used primarily to prevent or help to prevent Spam.
DomainKeys are used during the transmission of an email from server to server. Unlike SPF records, a part of the email is encrypted with a certificate. Essentially, a digital signature is added to each message that is sent. The message can then be verified to determine if it came from the domain it says it did. This signature also allows you to validate that the contents of the message were not altered since the time that the signature was placed.
The technology used is public key cryptography. Two keys are used when sending a message. A private key, that must remain hidden, is used to place a signature on the email and that signature is then verified by a public key, which can be widely published. If someone simply has a public key then they do not have enough information to create a fake signature. A message is signed using a private key and then published using a public key through the DNS. The signature attaches itself to the header of an email. Most subscribers will not see this header field.
How to setup
If you are not sure what you are doing then it is suggested that you contact your server admin or hosting provider for assistance.
You can learn more about DKIM and deploying DKIM From the following resources.